Focused Ion Beam (FIB) milling can be used during the course of Integrated Circuit (IC) development to modify circuits embodied within one or more layers of a semiconductor device. The FIB can be used to physically alter traces within the IC. In a FIB IC modification, very small holes, also referred to as vias, can be milled into the IC in order to expose conductive layers or traces that are buried under the surface of the IC. The depth of the vias may only be on the order of microns. Thus, knowing when the FIB milling process has reached the conductive layers or traces can be critical to the success of the modification. Undermilling, or completing a via before the conductive layer or trace is reached can result in a poor electrical connection. Additionally, overmilling, or continuing to mill after the conductive layer or trace has been reached, can destroy the conductor and defeat the intended modification.
Because of the precision required of the FIB milling process, various end-point detection methods are employed. One method employs visual feedback. The FIB milling process is periodically stopped and an operator views the milled hole to see if the conductive layer or trace has been reached. Typically, the operator uses some type of focused ion beam or scanning electron microscopy to view the end-point because of the small dimensions of the hole.
For low aspect ratio holes or relatively large diameter holes, where the depth of the hole is less than three times the diameter of the hole or the hole diameter is greater than approximately 0.75 microns, visual inspection may be an acceptable end-point detection method. However, visual inspection does not perform acceptably for high aspect ratio holes. In very high aspect ratio vias, where the depth of the hole is greater than or equal to approximately ten times the diameter of the hole, the depth of the hole may not allow an operator to see the bottom of the hole, or there may be insufficient contrast at the bottom of the hole for the operator to determine if the desired layer has been reached.
An alternative end-point detection method uses a FIB whose milling parameters are correlated to milling time or total ion dose. In such an end-point detection system, a FIB with a given beam current is used to mill various holes in a test IC. The depth of the holes are correlated against the time of milling or total ion dose to generate a characteristic curve. Curves can be generated for other beam currents, and curves can also be generated for different IC types, different process generations, and different IC foundries. A milling time needed to achieve a desired hole depth can then be approximated by examining the closest characteristic curve.
The numerous characteristic curves do not ensure accurate milling of a desired device because milling by time and current assumes a consistent milling process across multiple devices. The level of uniformity among devices may not always be sufficient to predict a milling depth based on characteristic curves. Drift in the device manufacturing process or in the FIB system can require complete re-characterization of the milling process.